Plesk 7.5.3 has no additional language packs. So you are getting this error - you had Swedish langpack, sure. What about upgrading - patch upgrades 7.5 version up to 7.5.3, so you had to upgrade 7.0.3 up to 7.5 and only after that - to 7.5.3. In 'readme' file they say, that you have to make a full backup, so, you can restore your Plesk from it. Or there were no backup?